Laura Nielsen for Frontier Scientists –
A grizzly bear chowing down on salmon, berries, ground squirrels, carrion, grasses and roots isn’t just hungry- he’s on a mission. During winter months while humans in cold climes are shoveling snow, most brown bears like the grizzly are sleeping.
This isn’t ‘true’ hibernation; the bear’s temperature drops to only about 88 °Farenheight from a standard 99 °F maintained during waking months. Some call it ‘denning’. In contrast, most small mammals which hibernate can reach a core body temperature of below 40°F, with a few species even dropping their temperature to below freezing. Still, bears exhibit a slowed metabolic rate (torpor), sparse oxygen consumption and low core body temperatures, just like other hibernators.
Ensconced in his den, a brown bear can sleep right through the winter. Unlike smaller hibernators, he needn’t awaken periodically to consume water or food, urinate or defecate. Bears in northern Alaska may remain asleep in their dens for eight full months, while some warmer southern parts of the state (i.e. Kodiak) have bear populations which never den. If a pregnant female does den, she does so much earlier than adult males. A mother will awaken during January or February to give birth to 1-4 cubs, most often twins. Cubs remain awake and nurse while their mother goes back to sleep until it’s time to emerge.
Bears exhibit useful traits while denning. With lowered metabolic rate, body temperature, slow breathing and a truly sparse heartbeat, they survive through harsh winters in a state of sleep. By burning fat stored up during the fall months, these creatures gain calories and maintain a good water balance. They have no need to awaken and drink water. Bears don’t experience degenerative bone loss, despite inactivity. Their bodies break down muscle and even organ tissue for protein. And unlike humans, their bodies are able to restore lost organ tissue (using nitrogen from urea to create new protein). Also unlike humans, the extremely high cholesterol levels which bears attain during fall months don’t induce cholesterol gallstones or hardened arteries. Impressive in themselves, the abilities which bears have evolved open doors for humans.
With modern-day science, can we harness the power of denning and hibernation? Hibernation could be used to stabilize patients who have experienced a stroke or heart attack, or are suffering from dangerously high fevers. It could extend the viability of transplant organs. In war, an induced state of hibernation could preserve wounded soldiers being moved to a hospital. Looking forward, the space industry could use hibernation like a sort of cryostasis. (It’s not so futuristic… NASA has already sponsored related research.)
Scientists at the University of Alaska Fairbanks took Arctic ground squirrels- creatures which already experience hibernation- and successfully aroused them then re-induced hibernation. An impressive feat. The researchers created a substance which mimics adenosine molecules. Adenosine is found in most animals. When it binds to receptors in the brain it causes drowsiness. Caffeine and caffeine-like substances bind to the same receptors and block adenosine, disallowing adenosine from causing torpor. The University of Alaska Fairbanks team successfully awoke hibernating ground squirrels with a caffeine-like substance. They then administered a substance to stimulate adenosine receptors in both non-hibernating and previously-hibernating ground squirrels. For the animals roused from hibernation, this re-induced hibernation. It worked more reliably on animals that had been in deep hibernation. Successfully inducing hibernation in animals is one step closer to- hopefully- one day being able to induce it in humans.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
FrontierScientists is inviting the public to send in their own photos of grizzly bears, which will be posted on the website. Submit photos in 300 dpi jpegs on this page: Readers Invited to Submit Bear Photos. Below, watch our vodcast ‘Denali’s Grizzly Population’, part of a FrontierScientists series on Grizzly bears in Alaska.
References:
Alaska Department of Fish and Game. Species Profile: Brown Bear (Ursus arctos). http://www.adfg.alaska.gov/index.cfm?adfg=brownbear.main
NOVA Online. Tyson, Peter. Bear Essentials of Hibernation. http://www.pbs.org/wgbh/nova/nature/bear-essentials-of-hibernation.html
Institute of Arctic Biology, Universtity of Alaska Fairbanks. Scientists identify hibernation-inducing signaling mechanism. http://www.iab.uaf.edu/news/index.php?newsrel=95
